Exciter for generator

ABSTRACT

An excitation apparatus for a generator includes an AC/AC inverter having a voltage holding function and connected to an output line of an AC generator. The apparatus includes an AC/DC converter, a capacitor connected to an output of the AC/DC converter, and a DC/AC converter connected to the capacitor and excites a field winding of the generator with an AC/DC converter controllable with a current and connected to the output of the AC/AC inverter. Even when a voltage of the output line is lowered due to a failure in a system, since energy stored in the capacitor is discharged and the voltage is maintained for the time being, the excitation current of the generator can be kept at a sufficiently high value.

TECHNICAL FIELD

The present invention relates to an excitation apparatus for controllingan excitation current that is supplied to a generator.

BACKGROUND ART

In general, of generators for supplying electric power to either acommercial power line or a large-scale house power generation line, thefollowing type excitation apparatus is called as a self-excitation typeexcitation apparatus and is widely used: an excitation apparatus inwhich a current is supplied to a field winding of a generator usingelectric power outputted from this generator.

In this case, since an excitation current needs to be controlled so asto control a generated voltage, generally speaking, the generatedvoltage is rectified via a controllable AC/DC converter, and a DCcurrent is supplied to a field winding of a generator.

Recently, since, for example, power transmission distance has increased,it is required for a generator to generate a higher ceiling voltage thanthat of the conventional generator. The reason why the powertransmission distance is increased is given as follows. As capacity of asingle generator is increased, it is practically difficult to install apower station in the vicinity of a power consuming place. As apparentfrom the foregoing fact, large output currents and quick responseperformance with respect to excitation apparatuses (referred to as a“high initial response excitation system”) are required.

Currently, static type excitation apparatuses are normally utilized asthis high initial response excitation system. This static typeexcitation apparatus has a superior response characteristic. However, avoltage drop occurring in the primary side of the excitation apparatusis likely to appear as a lowering of output voltage, especially loweringof a ceiling voltage. Since there is a small margin in output voltages,this static type excitation apparatus must be designed by employingextra power to make allowances for this voltage drop, resulting in awaste of performance.

A description will now be made of a conventional static type excitationapparatus.

FIG. 15 is a diagram showing the conventional static type excitationapparatus disclosed in Japanese Patent Application Laid-open No. Sho64-5400. In this drawing, reference numeral 1 denotes a generator;reference numeral 12 denotes a field winding of the generator 1;reference numeral 2 represents an output line of the generator 1;reference numeral 3 denotes a circuit breaker provided in the generatoroutput line 2; and reference numeral 4 denotes a system bus throughwhich this generator 1 supplies electric power.

Also, reference numeral 5 denotes an excitation transformer connected tothe output line 2 of the generator; reference numeral 6 shows a PT fordetecting an output voltage of the generator 1; and reference numeral 8represents an initial excitation circuit used when the generator 1 isfirst initiated (namely, when the generator is initiated under such acondition that supply of electric power is not received from the systembus 4).

Also, reference numeral 9 denotes a generator voltage adjuster forcontrolling a thyristor rectifier 10 in such a manner that the outputvoltage of the PT6 is kept to a predetermined value to control a currentof the field winding 12; reference numeral 11 indicates a field switchfor protecting a field circuit; and reference numeral 37 denotes avaristor for protecting the field circuit against an overvoltage.

Next, operation of the excitation apparatus of FIG. 5 will now bedescribed with reference to the drawings.

In such a case that while the generator 1 generates the voltage definedin a predetermined range, a load (not shown) connected to the system bus4 is operated under normal condition and a current defined within thenormal range flows, the voltage appearing on the output line 2 of thegenerator 1 is fed back to the generator voltage adjuster 9 by the PT6.The generator voltage adjuster 9 compares a set voltage value (notshown) with the voltage of the PT6, and controls the current of thefield winding 12 with the thyristor rectifier 10 in such a manner that adifference between the set voltage value and the PT6 voltage is reduced.

Assuming now that a short circuit, an earth fault, and the like happento occur in either the system bus 4 or the load (not shown) connected tothis system bus 4, the voltage of the system bus 4 is lowered, andfurthermore, the voltage appearing on the output line 2 of the generator1 is also lowered. As a result, the generator voltage adjuster 9controls the firing angle of the thyristor rectifier 10 in such a mannerthat a larger current may flow through the field winding 12. However,since the voltage which is applied via the excitation transformer 5 tothe thyristor rectifier 10 is also lowered, even when the firing angleis controlled at the maximum angle, the high voltage which could beoriginally produced by the thyristor rectifier 10 cannot be produced,and the current is not sufficiently increased.

In other words, the voltage variation occurring on the AC input side ofthe thyristor rectifier 10 may give no less and, moreover, instantadverse influence to the maximum outputtable voltage of this thyristorrectifier 10. When the AC input voltage is lowered, the maximumoutputtable voltage is immediately lowered. As a consequence, in acritical case, the system voltage is and, the necessary excitationvoltage cannot not output. Apparently, this drawback may be solved ifsuch an excitation apparatus having considerable extra capacity isdesigned allowing in advance for the voltage drop in the output line 2.If so, then the resulting excitation apparatus is increased in size andis not economical.

The present invention has been made to solve the above-describeddrawback of the conventional excitation apparatus, and therefore,provides an excitation apparatus for a generator, capable of applying anexcitation voltage and also an excitation current, which are required tooutput a necessary ceiling voltage even when a voltage of the outputgenerator is lowered due to a failure occurring in a power distributionsystem.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention, there is provided anexcitation apparatus for a generator, which comprises an AC/AC inverterhaving a voltage holding function and connected to an output line of anAC generator; and an AC/DC converter controllable with a current andconnected to an output side of this AC/AC inverter, a DC output side ofthis AC/DC converter being connected to a field winding of the ACgenerator.

According to another aspect of the present invention, there is providedan excitation apparatus for a generator, wherein the AC/AC inverterhaving the voltage holding function comprises: a capacitor connected tothe AC/DC converter and the DC output side of the AC/DC converter; and aDC/AC converter connected to the DC output side of the AC/DC converter.

According to still another aspect of the present invention, there isprovided an excitation apparatus for a generator, which comprises anAC/DC converter controllable with a current and connected to an outputline of an AC generator by a connection cable having an impedance Z, aDC output side of this AC/DC converter being connected to a fieldwinding of the AC generator, characterized by including an AC/ACinverter having a voltage holding function, the output terminal of whichis connected to the connection cable having the impedance Z on the sideof the AC/DC converter.

According to a further aspect of the present invention, there isprovided an excitation apparatus for a generator, wherein the AC/ACinverter having the voltage holding function comprises: an AC/DCconverter connected to another power supply line different from theoutput line of the AC generator; a DC/AC converter connected to a DCoutput side of this AC/DC converter; and a capacitor connected to anoutput side of the AC/DC converter.

According to a still further aspect of the present invention, there isprovided an excitation apparatus for a generator, which furthercomprises instead of the AC/AC inverter having the voltage holdingfunction, a bidirectional AC/DC converter and a capacitor connected to aDC side of this bidirectional AC/DC converter.

According to another aspect of the present invention, there is providedan excitation apparatus for a generator, wherein a series circuitconstructed of a semiconductor switch and a capacitor is connected inparallel to the field winding.

According to still another aspect of the present invention, there isprovided an excitation apparatus for a generator, which furthercomprises a charging circuit connected via a charging apparatus to acapacitor.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 1 of the present invention.

FIG. 2 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 2 of the present invention.

FIG. 3 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 3 of the present invention.

FIG. 4 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 4 of the present invention.

FIG. 5 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 5 of the present invention.

FIG. 6 is a circuit diagram for showing such a case that a modificationis made on the circuit arrangement of the excitation apparatus for thegenerator of FIG. 5.

FIG. 7 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 6 of the present invention.

FIG. 8 is a circuit diagram for showing such a case that a modificationis made of the circuit of FIG. 7.

FIG. 9 is a circuit arrangement diagram of an excitation apparatus for agenerator according to an embodiment 7 of the present invention.

FIG. 10 is a diagram showing a partially detailed circuit arrangement ofFIG. 9.

FIG. 11 is a diagram explaining operations of the circuit in FIG. 10.

FIG. 12 is a diagram showing a modification example of the partialcircuit in FIG. 10.

FIG. 13 is a diagram showing a modification example of FIG. 12.

FIG. 14 is a diagram for indicating a modification of FIG. 12.

FIG. 15 is a circuit arrangement diagram of a conventional excitationapparatus for a generator.

BEST MODE FOR CARRYING OUT THE INVENTION EMBODIMENT 1

Referring now to FIG. 1, an embodiment 1 of the present invention willbe described below.

FIG. 1 is a circuit arrangement diagram showing an excitation apparatusfor a generator according to the present invention. In this drawing,reference numeral 1 denotes a generator; reference numeral 12 denotes afield winding of the generator 1; reference numeral 2 represents anoutput line of the generator 1; reference numeral 3 denotes a circuitbreaker provided in the generator output line 2; and reference numeral 4denotes a system bus through which this generator 1 supplies electricpower.

Also, reference numeral 5 denotes an excitation transformer connected tothe output line 2 of the generator; reference numeral 6 shows a PT fordetecting an output voltage of the generator 1; and reference numeral 8represents an initial excitation circuit used when the generator 1 isfirst initiated (namely, when generator is initiated under such acondition that a supply of electric power is not received from thesystem bus 4).

Also, reference numeral 9 denotes a generator voltage adjuster forcontrolling a thyristor rectifier 10 in such a manner that the outputvoltage of the PT6 is kept to a predetermined value to control a currentof the field winding 12; reference numeral 11 indicates a field switchfor protecting a field circuit; and reference numeral 37 denotes avaristor for protecting the field circuit from an overvoltage. It shouldbe noted that the thyristor rectifier 10 corresponds to a currentcontrollable AC/DC converter as defined in the present invention.

Reference numeral 13 indicates an AC/DC converter for converting ACderived from the excitation transformer 5 into DC. This AC/DC convertermay be replaced by a simple rectifying apparatus, or a controlledrectifier capable of controlling a voltage. Reference numeral 14 shows acapacitor (hereinafter, referred to as link capacitor) having a largecapacitance connected to the DC output side of the AC/DC converter 13,and this capacitance will be explained later. Reference numeral 15denotes a DC/AC converter for converting the DC output voltage of theAC/DC converter 13 into an AC voltage. It is preferable to employ such aDC/AC converter capable of outputting a constant voltage. This ACvoltage and the frequency thereof are such an AC voltage/frequency bywhich the thyristor rectifier 10 can be operated without any problem.The AC/DC converter 13, the capacitor 14, and the DC/AC converter 15constitute an AC/AC converter 29 having a voltage holding function asdefined in the present invention.

In general, of the cases in which the voltage of the generator 1 islowered, conceivable ones include a failure occurring on the side ofsystem 4, or in the output line 2 of the generator. In the case of thesystem having the commercial frequency, the recovery time by the plantafter these failures have occurred by the protection apparatus is 4 to 6cycles.

As the excitation apparatus for the generator, this generator mustoutput the ceiling voltage so as to mitigate the voltage reduction ofthe system during at least the above-explained 6 cycles until thefailure can be recovered. The output voltage of the AC/DC converter 13is also lowered in conjunction with the voltage reduction of the system.However, in this case, since electric charge stored in the linkcapacitor 14 is discharged, the voltage reduction may be slowed.

Assuming now that an apparent resistance as viewed from the linkcapacitor 14 to the side of the DC/AC converter 15 is “R” and thecapacitance of the link capacitor is “C”, this voltage reduction isadvanced based upon a time constant of RC. Assuming now that theoperating voltage margin of the thyristor rectifier 10 is 20% (namely,if there is margin of A% in voltage reduction on the AC input side ofthyristor rectifier 10), the capacitance of the link capacitor may beselected by satisfying the following relationship:

RC×(A/100)>time period of 6 cycles, where:

R is nearly equal to:

(voltage when link capacitor 14 is operated under normalcondition)/(input current when DC/AC converter 15 is operated undernormal condition).

EMBODIMENT 2

A circuit arrangement of an embodiment 2 of the present invention isshown in FIG. 2.

It should be noted that the same reference numerals shown in FIG. 1 willbe employed as those for denoting the same, or corresponding circuitportions illustrated in this drawing, and detailed explanations thereofare omitted. Reference numeral 16 shows an auxiliary transformerprovided on the input side of the AC/DC converter 13, and referencenumeral 17 shows a step-up transformer provided on the output side ofthe DC/AC converter 15. Also, reference numeral 31 denotes an equivalentimpedance of a cable used to connect the excitation transformer 5 to thethyristor rectifier 10. The auxiliary transformer 16 is connected to theside of the excitation transformer 5 of the above-described equivalentimpedance 31, whereas the step-up transformer 17 is connected to theside of the thyristor rectifier 10 of the above-described equivalentimpedance 31. Reference numeral 27 is a monitor apparatus for monitoringthe output voltage of the excitation transformer 5, and for producing asignal 51 used to control the DC/AC converter 15 when this outputvoltage is extremely lowered. This signal 51 causes, when the system isoperated under normal condition, the output current of the DC/ACconverter 15 become nearly zero, or the operation of the DC/Ac converterto be.

In the case of FIG. 2, while the system is operated under normalcondition (namely, there is no abnormal condition in system), thevoltage is directly applied from the excitation transformer 5 to thethyristor rectifier 10, whereas when the voltage appearing on the outputline 2 is lowered, the voltage is applied from the link capacitor 14 viathe DC/AC converter 15 to the thyristor rectifier 10. There is noadverse influence even when an AC/AC inverter circuit 29 having thevoltage holding function during the normal operation is out of order. Itis needless to say that the DC/AC converter 15 of FIG. 2 is arranged insuch a way that this DC/AC converter 15 can output a voltage having aphase coincident with that of the output voltage of the excitationtransformer 5.

EMBODIMENT 3

A circuit arrangement of an embodiment 3 of the present invention isshown in FIG. 3.

In this drawing, although the monitor apparatus 27 is omitted, thismonitor apparatus 27 is employed in a similar manner to that of FIG. 2.Reference numeral 52 denotes a separately excited power supply apparatus(for example, house AC power supply installed in a power station). Sincethe power supply of the AC/DC converter 13 is received from thisseparately excited power supply apparatus, the below-mentioned meritsare achieved:

1) Since this separately excited power supply may be used in the firstinitial excitation when the generator is initiated, the initialexcitation circuit 8 drawn in FIG. 2 of the embodiment 2 may be omitted.

2) When the voltage of the house AC power supply 52 is set to be higherthan the voltage of the excitation transformer 5, or this voltage of thehouse AC power supply 52 is boosted by the step-up transformer 17provided on the side of the AC/AC inverter circuit 29, a higher ceilingvoltage can be produced.

3) Since only the output side of the AC/AC converter circuit 29 isconnected on the output side of the excitation transformer 5, theadverse influence caused by trouble in the AC/AC converter circuit 29can be hardly given, and thus the reliability can be improved.

EMBODIMENT 4

FIG. 4 shows an arrangement of an excitation apparatus for a generatoraccording to an embodiment 4 of the present invention.

In this drawing, reference numeral 35 denotes a bidirectional DC/ACconverter operable along both directions, namely a DC/AC convertercapable of performing a regenerative operation.

During the normal operation, electric power supplied from the step-uptransformer 17 is converted into DC by the bidirectional DC/AC converter35 so as to charge the link capacitor 14. Then, when a voltage appearingacross a secondary winding side of the excitation transformer 5 islowered due to a failure occurring in the system, the energy of the linkcapacitor 14 is returned to the step-up transformer 17 by way of theregenerative control, so that the voltage is maintained. Since theimpedance 31 is included, though not shown, in the excitationtransformer 5 and the wiring lines connected to the secondary windingthereof, there is no risk that the output voltage of the DC/AC converter35 will be extremely lowered by the dropping voltage of the system, andtherefore, a sufficient voltage can be applied to the thyristorrectifier 10.

In response to the signal 51 indicative of trouble in the system, theDC/AC converter 35 switches the normal DC constant voltage outputoperation to the AC constant voltage output operation along the reversedirection. Alternatively, for example, the DC/AC converter 35 may beoperated by utilizing as a trigger a voltage reduction appearing in theprimary winding of the step-up transformer 17.

Since the excitation apparatus for the generator shown in FIG. 4requires a small number of converters, as compared with those of theexcitation apparatus indicated in FIG. 1 to FIG. 3, this excitationapparatus is economical and also is improved in reliability.

EMBODIMENT 5

FIG. 5 shows an arrangement of an excitation apparatus for a generatoraccording to an embodiment 5 of the present invention.

In this drawing, reference numeral 18 denotes a switch S1 with the useof a high-speed switching element (for example, a self-extinction typeelement such as a so-called “GTO”). Reference numeral 19 denotes anotherswitch S2 similarly employing a high-speed switching element. Thisswitch S2 and the switch S1 have an antiparallel-connection. Referencenumeral 20 denotes a charging capacitor; symbol “Vf” denotes a voltageindicative of the output voltage of the thyristor rectifier 10, for thesake of an easy explanation; and symbol “Vc” denotes a terminal voltageof the capacitor 20.

While the voltage relationship of Vc<Vf is maintained under normaloperation, the switch S1 (18) is closed so as to charge the capacitor20.

When a failure occurs, the switch S2 (19) is turned ON so as todischarge the energy of the capacitor 20 to the field winding 12. Withexecution of the above-described operations, since the field current ofthe generator is directly supplied from the capacitor 20 when theceiling voltage is produced while the failure occurs, there is noadverse influence caused by a delay in firing angle control for thethyristor rectifier 10, and the immediate action of the excitationapparatus can be improved. In this case, the switch S1 (18) may bereplaced by a simple diode. Also, a limiting resistor may beseries-connected to the switch S1 (18) in order to avoid a delayoccurring when the field current is increased by the presence ofcapacitor 20 in charging the capacitor 20.

Also, FIG. 6 shows a circuit arrangement of another excitation apparatusfor a generator, from which the varistor 37 is omitted, since a smallnumber of circuits are added to the circuit shown in FIG. 5. In thisdrawing, reference numeral 23 is a switch S3 (23) using a high-speedswitching element, and this switch is connected to one terminal of thecapacitor 20. Reference numeral 24 denotes a resistor series-connectedto the switch S3 (23). When the field voltage Vf is extremely increased,this increased field voltage Vf is detected by a not-shown voltagedetecting circuit. Then, as a first stage, the switch S1 (18) is turnedON, so that the voltage is absorbed by the capacitor 20. If the voltageis further increased even after such a voltage is absorbed by thecapacitor 20, then the switch S3 (23) is closed, so that the surgeenergy is discharged via the resistor 24 so as to suppress theovervoltage. With this arrangement, the varistor 37 can be omitted.

EMBODIMENT 6

FIG. 7 shows a circuit arrangement according to an embodiment 6 of thepresent invention.

In the case of the system of the embodiment 5 shown in FIG. 5, since thecharge current to the capacitor 20 is shunted from the circuit forenergizing the field system, there is an adverse influence given to theresponse performance of the field control. In accordance with the systemof the embodiment 6 shown in FIG. 7, this adverse influence can beprevented.

In the drawing, reference numeral 21 indicates a charging circuit bywhich an AC input shunted from the output of the excitation transformer5 is rectified so as to supply a DC current used to charge the capacitor20. Reference numeral 22 denotes a diode. Apparently, if the chargingcircuit 21 has (as most charging circuit do) a function capable ofblocking the entry of the DC derived from the output side thereof, thediode 22 may be omitted.

Under normal conditions, the switch S2(19) is turned OFF so as to chargethe capacitor 20 with the charging circuit 21, and the thyristorrectifier 10 can control the current of the field winding 12 withouthaving any adverse influence caused by the capacitor 20. When a failureoccurs, the switch S2(19) is turned ON so as to discharge the electroniccharge of the capacitor 20 to the field winding 12.

It should be noted that when the power is supplied to the chargingcircuit 21 from the house power supply of the power station, as shown inFIG. 8, the charging circuit 21 may serve also as the initial excitationapparatus 8 (namely, if switch S2 (19) is turned ON under a state inwhich the thyristor rectifier 10 is stopped, then output of chargingcircuit 21 may be directly supplied to field winding 12 from a separatepower source 52) and thus, the initial excitation circuit 8 is no longerrequired.

EMBODIMENT 7

FIG. 9 shows a circuit arrangement of an excitation apparatus accordingto an embodiment 7 of the present invention.

In this drawing, reference numeral 25 denotes a DC/DC converter foroutputting an output voltage of the AC/DC converter 13 into the fieldwinding 12 as a voltage controlled in accordance with an instruction ofthe generator voltage adjuster 9.

Since an excitation current is directly outputted from the DC/DCconverter 25, the thyristor rectifier 10 is no longer needed.

Operation of the apparatus will next be described.

A voltage at the link capacitor 14 is continuously charged to asufficiently high value by the AC/DC converter 13.

Even when a voltage appearing at the secondary winding side of theexcitation transformer 5 is lowered by a failure occurring in thesystem, since the link capacitor 14 can supply a voltage higher than, orequal to a minimum DC voltage required for the DC/DC converter 25 tooutput a necessary voltage for the time being, the DC/DC converter 25can generate a desired ceiling voltage.

The circuit of the DC/DC converter 25 is comprised of, for example, aself-extinction type high-speed switch SW4 (26) and a diode 27, as shownin FIG. 10. A field current may be produced by turning ON/OFF the switchSW4 (26) while the excitation apparatus is operated under normalcondition, and a waveform of this field current is shown in FIG. 11.Symbol “If” in this drawing denotes the field current.

The circuit of FIG. 10 is arranged as shown in this drawing, while acenter tap is provided in the AC/DC converter 13 (namely, outputvoltages are +, 0,− at 3 terminals) as illustrated in FIG. 12. As aresult, it is well known that the convergence speed of the field currentwhen the switch SW1 is turned OFF can be improved. In this drawing, acapacitor C1 (20) corresponds to the capacitor (14) of FIG. 10. However,in this circuit, the output voltage becomes (−N) when the switch SW1 isturned OFF. Thus, since there is such a trend that the mean value of theoutput voltages is lowered, a flywheel switch SW3 (30) may befurthermore employed, as shown in FIG. 13. In such a case that a largechange control of the field current is not carried out, the flywheelswitch SW3 is continuously turned ON. In the case that the current isrequired to be rapidly reduced, the switch SW1 is turned OFF, and at thesame time, the flywheel switch SW3 is also turned OFF.

With execution of the above-described operation, as the usual outputvoltage, the high voltage can be readily outputted by way of theswitching control between +P and 0V, whereas the current can be rapidlyreduced by way of the switching control between +P and −N, if required.

A circuit of FIG. 14 shows a circuit arrangement for more smoothlyswitching the control mode executed in FIG. 13.

While the excitation apparatus is operated under normal condition, boththe switch SW13 and the switch SW14 are turned OFF, whereas both theswitch SW1 and the switch SW12 are turned ON, and a voltage of +P isapplied to the field system. Thereafter, when the switch SW1 is turnedOFF and the switch SW12 is turned ON, the field voltage becomes 0V.

In rapid demagnetization, while the switch SW1 and the switch SW12 areturned OFF, a negative voltage is applied by a loop constructed of thediodes D5 and D6 so as to attenuate a current. The attenuation speed canbe arbitrarily controlled by varying the OFF time of the switch SW1 andalso the OFF time of the switch SW12.

INDUSTRIAL APPLICABILITY

The excitation apparatus for the generator, according to the presentinvention, can be utilized not only for ac generators operated in acommercial power line but also for any type of generators, as long as itis a generator excited by DC.

What is claimed is:
 1. An excitation apparatus for a self-excitedgenerator comprising: an AC/AC converter having an input side coupled toan output line of an AC generator and comprising: an AC/DC converterhaving an input side coupled to the output line of the AC generator, aDC/AC converter having an input side connected to an output side of theAC/DC converter, and a capacitor connected at a first terminal to theoutput side of the AC/DC converter and to the input side of the DC/ACconverter, for storing energy; a thyristor having a gate, the thyristorbeing connected between an output side of the AC/AC converter and afield winding of the AC generator; a potential transformer coupled tothe output line of the AC generator; and a generator voltage adjusterconnected between the potential transformer and the gate of thethyristor for controlling switching of the thyristor, the AC/ACconverter storing electrical energy that is supplied to the thyristorwhen voltage on the output line decreases, increasing the currentsupplied to the field winding to counteract decreasing output linevoltage.
 2. The excitation apparatus according to claim 1, furthercomprising: a connection cable having an impedance Z, coupled at a firstend to the output line of the AC generator and connected at a second endto the DC/AC converter; and first and second transformers respectivelycoupling the input and output sides of the AC/AC converter to the firstend of the connection cable having the impedance Z and to the thyristor.3. An excitation apparatus for a generator comprising: a bidirectionalAC/DC converter having an external terminal coupled to an output line ofan AC generator, an internal terminal, and a capacitor connected to theinternal terminal; a thyristor having a gate, the thyristor beingconnected between the external terminal of the bidirectional AC/DCconverter and a field winding of the AC generator; a potentialtransformer coupled to the output line of the AC generator; and agenerator voltage adjuster connected between the potential transformerand the gate of the thyristor for controlling switching of thethyristor, the bidirectional AC/DC converter storing electrical energythat is supplied to the current-controlled AC/DC converter when voltageon the output line decreases, increasing the current supplied to thefield winding to counteract decreasing output line voltage.
 4. Anexcitation apparatus for a self-excited generator comprising: an AC/ACconverter having an input side connected to a power source separate froman output line of an AC generator, the AC/AC converter comprising: anAC/DC converter having an input side coupled to the output line of theAC generator, a DC/AC converter having an input side connected to anoutput side of the AC/DC converter, and a capacitor connected at a firstterminal to the output side of the AC/DC converter and to the input sideof the DC/AC converter, for storing energy; a connection cable having animpedance Z, coupled at a first end to the output line of the ACgenerator and connected at the second end to an output side of the AC/ACconverter; a thyristor having a gate, the thyristor being connectedbetween an output side of the AC/AC converter and a field winding of theAC generator; a potential transformer coupled to the output line of theAC generator; and a generator voltage adjuster connected between thepotential transformer and the gate of the thyristor for controllingswitching of the thyristor, the AC/AC converter storing electricalenergy that is supplied to the thyristor when voltage on the output linedecreases, increasing the current supplied to the field winding tocounteract decreasing output line voltage.
 5. The excitation apparatusfor a generator according to claim 4, including a transformer couplingthe output side of the AC/AC converter to the AC/DC converter.
 6. Anexcitation apparatus for a generator comprising: a thyristor having agate, the thyristor being coupled at an input side to an output line ofan AC generator and connected at an output side to a field winding ofthe AC generator; a potential transformer coupled to the output line ofthe AC generator; a generator voltage adjuster connected between thepotential transformer and the gate of the thyristor for controllingswitching of the thyristor, and a charge storage converter connected inparallel with the field winding of the AC generator, the charge storageconverter including a series circuit of a first semiconductor switch anda capacitor connected in series with each other, the series circuitbeing connected in parallel with the field winding of the AC generator,and a second semiconductor switch connected in anti-parallel with thefirst semiconductor switch, the first semiconductor switch conducting tostore electrical energy in the capacitor and the second semiconductorswitch conducting to supply stored electrical energy from the capacitorto the field winding when the current supplied to the field windingdecreases, thereby counteracting a decrease in voltage on the outputline of the AC generator.
 7. An excitation apparatus for a generatorcomprising: a thyristor having a gate, the thyristor being coupled at aninput side to an output line of an AC generator and connected at anoutput side to a field winding of the AC generator; a potentialtransformer coupled to the output line of the AC generator; a generatorvoltage adjuster connected between the potential transformer and thegate of the thyristor for controlling switching of the thyristor, and acharge storage converter connected in parallel with the field winding ofthe AC generator, the charge storage converter including a seriescircuit of a semiconductor switch and a capacitor connected in serieswith each other, the series circuit being connected in parallel with thefield winding of the AC generator, and a charging circuit connected atan output side to a junction of the semiconductor switch and thecapacitor for storing electrical energy in the capacitor, thesemiconductor switch supplying stored electrical energy from thecapacitor to the field winding when the current supplied to the fieldwinding decreases, thereby counteracting a decrease in voltage on theoutput line of the AC generator.
 8. The excitation apparatus for agenerator according to claim 7, wherein the charging circuit is coupledat an input side to the output line of the AC generator.
 9. Theexcitation apparatus for a generator according to claim 7, wherein thecharging circuit is coupled at an input side to a power source separatefrom the output line of the AC generator.